1. Field of the Invention
The present invention relates to polarizing monochromator for selecting linearly polarized light having a certain wavelength range, and more specifically, to an improvement of a stray-light rejection system thereof.
2. Prior Art
With the increase in the degree of semiconductor integration, the wavelength of light sources used in optical lithography has been shortened to the vacuum ultraviolet range, as a means for improving resolution. As the popularity of polarized illumination using s-polarization to increase the degree of contrast and exposure to polarized light sources increases, polarizing monochromator for producing monochromatic, linearly polarized light with a high throughput down to the vacuum ultraviolet range have become indispensable for analysis of materials.
A general configuration used to produce monochromatic, linearly polarized light lets light output from a general monochromator pass through a polarizer. This type of configuration, however, would result in a loss due to the throughput of the polarizer. In order to avoid such a loss and stray light, a monochromator based on a double monochromator design has been used. One monochromator performs wavelength dispersion, and the other monochromator performs the functions of a wavelength dispersion element and a polarizer. This polarizing monochromator provides monochromatic, linearly polarized light by using a crystal prism functioning as a wavelength dispersion element and a polarizer simultaneously. In other words, linearly polarized light can be obtained through separation of ordinary light and extraordinary light by double refraction in the crystal prism.
The crystal prism used as the wavelength dispersion element has much higher wavelength dispersion on shorter wavelength side. Accordingly, the slit width can be widened, which results in increased optical throughput and provides measurement data with an improved signal-to-noise ratio.
However, because of the small separation angle between ordinary light and extraordinary light, the polarizing monochromator described above cannot sufficiently eliminate one linearly polarized light component working as a stray light component. For instance, the slit width cannot be widened at longer wavelengths because of small dispersion. If the slit width is widened at the expense of wavelength resolution, stray light would get in. A slit width widened for short-wavelength light would also cause stray light to get in.